Astronomers gain new tape measure for the universe

Using binary stars as a standard for reference, astronomers have measured the distance to our neighbouring galaxy, the Large Magellanic Cloud (LMC), more accurately than ever before.

And improving the accuracy of this measurement has a knock-on effect, improving knowledge of the Hubble Constant, the rate of expansion of the universe, and therefore of of the mysterious dark energy that is causing the expansion to accelerate.

"I am very excited because astronomers have been trying for a hundred years to accurately measure the distance to the Large Magellanic Cloud, and it has proved to be extremely difficult," says Wolfgang Gieren of the Universidad de Concepción, Chile. "Now we have solved this problem by demonstrably having a result accurate to two percent."

Astronomers survey the scale of the universe by first measuring the distances to close-by objects and then using them as standard candles to pin down distances further and further out into the cosmos.

Stars in the Large Magellanic Cloud are used to fix the scale for more remote galaxies, meaning that measuring its distance is particularly important.

Now, careful observations of a rare class of double star have now allowed a team of astronomers to deduce a much more precise value for the LMC distance: 163,000 light-years.

The astronomers managed this by observing rare close pairs of stars, known as eclipsing binaries. When one passes in front of the other, as seen from Earth, the total brightness drops.

By tracking these changes in brightness very carefully, and also measuring the stars' orbital speeds, it is possible to work out how big the stars are, their masses and other information about their orbits. When this is combined with careful measurements of the total brightness and colours of the stars, remarkably accurate distances can be found.

This method has been used before, but with hot stars - which requires certain assumptions to be made, and gives less accurate results. But now, for the first time, eight extremely rare eclipsing binaries where both stars are cooler red giant stars have been identified, which can yield much more accurate distance values - accurate to about two percent.

"We are working to improve our method still further and hope to have a one percent LMC distance in a very few years from now," says Dariusz Graczyk of Warsaw University Observatory. "This has far-reaching consequences not only for cosmology, but for many fields of astrophysics."